1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, or a multi-function apparatus having one or more capabilities of the above devices.
2. Description of the Related Art
In an image forming apparatus such as a copier, a printer, a facsimile machine, or a multi-function apparatus having one or more capabilities of the above devices, an image forming unit included in the image forming apparatus forms an image on a medium, such as a sheet, based on image data input from a scanner or an external personal computer. In this case, a sheet of a size corresponding to the input image data is normally fed from a sheet feed unit to the image forming unit.
However, when a sheet having a shorter size in the sheet conveyance direction than a desired size is set in the sheet feed unit, it can happen that two shorter-sized sheets are conveyed successively. When two shorter-sized sheets are conveyed continuously, the image to be formed on one larger-sized sheet is formed crossing the boundaries of the two shorter-sized sheets, and therefore, a toner image may be transferred starting from a leading end of the second sheet.
Normally, in an image forming apparatus employing a thermal fixing method in which the toner image is thermally fixed by using, for example, a fixing roller, part of the leading end of the sheet is a margin where the toner image is not transferred. Because the marginal part does not exert an adhesive force on the toner, separability of the sheet with respect to the fixing roller is assured. However, as a result of the two shorter-sized sheets being conveyed continuously, if the toner image is transferred from the leading end of the second sheet, the second sheet is not cleanly separated from but is instead wound around the fixing roller.
To cope with the above problem, JP-2007-121885-A, for example, discloses an image forming apparatus configured to temporarily stop a sheet feed roller immediately after the sheet has been conveyed up to half the length thereof set by the user in the conveyance direction, and thereafter, rotate the sheet feed roller again to convey the sheet. By thus controlling driving of the sheet feed roller, even though the length in the conveyance direction of the set-sheet is only half the length in the conveyance direction of the sheet to be originally set, a gap may be formed between a first sheet and a second sheet, thereby preventing two sheets being conveyed continuously without any space in between. By detecting the gap with a sensor, the image forming apparatus can recognize that two small-sized sheets have been conveyed continuously.
According to the method disclosed by JP-2007-121885-A, when the length in the conveyance direction of the erroneously-set sheet is half or less than half the length in the conveyance direction of the sheet to be originally set, a gap may be formed between the first sheet and the second sheet. However, when the length in the conveyance direction of the erroneously-set sheet is longer than half that of the sheet to be originally set, a gap may not be formed between the first sheet and the second sheet. Accordingly, there is such a problem that the continuous feeding of the erroneously set sheets cannot be detected. For example, when A3-sized sheets should have been set with its longer side along the sheet conveyance direction, assume that B4-sized sheets are instead mistakenly set with its longer side along the sheet conveyance direction. In this case, because the longer side of the B4-sized sheet is longer than half the longer side length of the A3-sized sheet, the erroneous setting can not be detected.
Even in this case, if a side fence capable of detecting a shorter side of the set-sheet is provided, the erroneous setting can be detected from the difference between the shorter-side length of the A3-sized sheet and that of the B4-sized sheet. However, provision of the side fence as a detection means to the sheet feed increases costs and makes the apparatus larger, and thus is not a practical option for low-end printers facing fierce cost competition.
Further, when using the usual contact-type feeler (swing lever common to small printers to detect a gap between the first and the second sheets, a 15 to 20 mm gap is required between sheets so that the feeler detects the gap between sheets. In order to secure that gap, the leading end of the sheet needs to have a bending portion longer than 2 to 4 mm for the normal sheet so as to align the conveyed sheet to be straight. As a result, the bending portion of the sheet interferes with conveyance guides to cause abnormal noise or damage such as creasing of the sheet. When using a thick sheet having greater rigidity, even though the sheet is controlled to be conveyed to have a larger bending amount, the sheet feed roller slips due to the rigidity of the sheet and the bending amount does not increase.